The present invention relates to the imaging arts It finds particular application in conjunction with CT scanners and will be described with particular reference thereto. It is appreciated, however, that the invention will also find application in conjunction with other types of devices in which x-rays or electromagnetic radiation is used to generate images.
In early x-ray tubes, electrons from a cathode filament were drawn at a high voltage across a vacuum to a stationary target anode. The impact of the electrons caused the generation of x-rays, as well as significant thermal energy. As higher power x-ray tubes were developed, the thermal energy became so large that extended use damaged the anode. Thus, ways to reduce or dissipate the thermal energy were required.
There are various generally accepted ways to transfer heat energy; namely, convection, conduction, and radiation. With reference to x-rays tubes, convection is ineffective due to the vacuum in which the anode is located. Thus, radiation and conduction remain the primary methods of heat exchange. Both conduction and radiation dissipate heat more slowly than it is generated.
A popular solution is to mount anodes rotatably in the vacuum. By rotating the anode, the thermal energy is distributed over a larger area. However, when the rotating anode tubes are operated for longer durations at high power, the thermal buildup can again damage the electrode. Radiation transfers heat slowly, more slowly than it is added during x-ray generation. Conduction removes heat more efficiently than convection or radiation. However, in a rotating anode x-ray tube the only conduction path is typically through a bearing on which the anode is mounted. Not only does the passage of heat through a bearing degrade it, but the conduction is still slower than the rate at which energy is added. The circulation of cooling fluid through the bearing causes numerous fluid and vacuum sealing difficulties.
Thus, the limited thermal cooling rates have led to duty cycle requirements which limit x-ray generation durations and increase the interval between successive operations. Initially, x-ray exposure times were relatively short, and the time between these exposures was relatively long. Long set-up times are typical today in many applications, e.g. x-rays for orthopedic or dental evaluation, single slice CT scans and the like. Short exposure times coupled with subject repositioning provide the time for the anode to transfer the heat generated. Thus, duty cycle restrictions in these applications are rarely a problem. However, with the advent of the CT scanner, particularly spiral and volume CT scanners, the duty cycle restrictions are again limiting the rapidity with which repetitions can be performed.
Aside from imposed duty cycles, present x-ray tubes also restrict operations periodically due to failure conditions. For example, most all present x-ray machines, including commercially available CT scanners, contain a single x-ray tube. When the tube fails, the machine is inoperable until a replacement tube can be installed. However, because these tubes are very expensive, `spares` are usually not kept on hand. Moreover, x-ray tubes usually are replaced only by specialized, trained personnel. Purchase and installation of the replacement tube can take as long as several days. Thus, when this one component of a CT scanner fails, an expensive machine with tremendous diagnostic capabilities is idled.
Beyond single tube machines, multiple tube scanners such as disclosed in Franke U.S. Pat. No. 4,150,293; Franke U.S. Pat. No. 4,384,395; and Polacin et. al. U.S. Pat. No. 5,604,778 compound the failure problem. Multiple tube systems use a plurality of tubes simultaneously to shorten the amount of rotation required in order to obtain a complete image. However, these systems depend on all of the plurality of x-ray tubes being operational. Said another way, the multitube systems are only as reliable as the weakest tube, and the likelihood of failure increases by the number of tubes used.
Potentially more disruptive than complete tube failure is the arcing typically seen in x-ray tubes nearing the end of their useful lives. As a tube ages, its vacuum becomes harder to maintain, and as the vacuum is lost periodic arcing is observed. This arcing causes ions to be freed within the tube further fouling the vacuum. Moreover, following arcing the tube requires a `rest` time while the vacuum is reestablished after which the tube is ready to use again. Gradually the `off` times lengthen while the `on` times ebb. Notwithstanding the increased duty cycle times that these rests impose, aging tubes are not typically replaced as they begin to arc. Rather, the situation is allowed to deteriorate before tube replacement.
The present invention contemplates a new, efficient x-ray tube, CT gantry and method of use which overcomes the above referenced problems and others.